Nowadays, liquid crystals are used in various fields. Liquid crystals have features such as optical anisotropy and easiness of change in molecule orientation under the influence of an external field such as electric field or magnetic field. In particular, ferroelectric liquid crystals are driven by a torque which is generated as a result of interaction between spontaneous polarization and electric field. This driving power is very large so that ferroelectric liquid crystals are expected to find uses as materials of high-speed display devices. There are some dyes which exhibit superior electro-optic effect under ordinary electric field or photoelectric field, as well as excellent optical characteristics such as non-linear optical effect. These functional low molecular liquid crystals and dyes have possibilities of wide variety of molecular design and exhibit diversified optical characteristics but are difficult to fix because they have high levels of fluidity.
In conventional optical display devices, functional low molecular substance such as liquid crystal is fixed by being sandwiched between a pair of plates of glass. The use of such glass plates poses problems such as difficulties encountered in achieving improvements for greater flexibility, reduced weight and increased area. In order to avert from these problems, it has been considered to use polymers which are generally flexible, light-weight and easy to form in large areas, and methods have been proposed for fixing the functional polymers. Among various polymers, ferroelectric substances exhibit superior electric characteristics such as high-dielectric constant, high-speed switching performance, piezoelectricity, pyroelectricity, electrostrictivity and so forth. Therefore, composite films formed by liquid crystal and/or dye and polymer, formed by complexing a low molecular substance and a polymer both of which are functional, are considered to have various functions in addition to the electro-optic effects and non-linear optic effects inherently possessed by the starting materials. Thus, such composite films are expected to find uses as industrial materials for various devices such as optical devices, e.g., switches and memories, second harmonic generation of semiconductor laser, photo-bistable memory, photo-processor and so forth.
The following two processes have been known as processes for forming a composite film of liquid crystal and polymer.
(1) In the first process, a film is cast-formed by solvent evaporation method (conventional method) from a solution of a mixture of liquid crystal and a polymer. Namely, a polymer such as polyvinyl chloride and polycarbonate is dissolved in a solvent in which also is dissolved liquid crystal such as N-(4-ethoxybenzylidene)-4'-butyl aniline, butyl-4-(4-ethoxyphenoxycarbonyl)phenyl carbonate or the like, and the thus prepared solution is spread in a petri dish so as to allow the solvent to evaporate, whereby a film is formed.
(2) The second process is called water-surface developing method. A solution of mixture of liquid crystal and polymer is made to drip onto the water surface along a tank wall so that an ultra-thin film in formed by self-diffusion. A multilayer of thus obtained thin films is superposed to form a film.
Unfortunately, composite films thus formed from a functional low molecular substance such as liquid crystal or dye and polymer are still unsatisfactory in electric and optical characteristics due to non-uniform diameter of diffusion of the functional low molecular substance and non-uniform dispersion of the same.
The present inventors have proposed a composite film and a process for producing the film, wherein 65/35 mol % copolymer of vinylidene fluoride and trifluoroethylene is dissolved in a solvent which is a mixture of tetrahydrofuran and benzene, forming a porous VDF/TrFE film by making use of the difference in the boiling point and charging the liquid crystal. This method is reported in "PRE-PRINT OF LECTURE MEETING OF ASSOCIATION OF POLYMER ENGINEERING" (Vol. 36, No.8, pp. 2383-2385), published by the association of polymer engineering on Oct. 5, 1987. However, it has been difficult to obtain a porous film from the composite film produced by the reported method.